The present invention generally relates to a heat insulating panel and, more particularly, to a vacuum-packed, powder containing heat insulator.
Hitherto, as a heat insulating panel, glass fibers, rock wool, urethane foam, and others have been employed. Although both the glass fiber and the rock wool have a good heat resistance, they can not give a favorable heat insulating effect in view of the thermal conductivity being within the range of 0.03 to 0.05 Kcal/mh.degree. C. On the other hand, foamed plastics such as, for example, foamed polyurethane and foamed polystyrene are generally used as a low temperature retaining material for use in a refrigerator. While the foamed polyurethane has a thermal conductivity of 0.015 Kcal/mh.degree. C. at 24.degree. C., the state of art is such that the heat insulating property of the foamed polyurethane can no longer be improved. This situation is the same with the foamed polystyrene.
Apart from the above, as a heat insulating panel for use in a pressure vessel for accommodating liguefied petroleum gas or liquid nitrogen, there is also known a vacuum-packed, powder containing a heat insulator comprising a mass of foamed pearlite particles of bubble-like shape, 100 to 300 micrometers in average particle size, filled in a vacuum-packed space defined in of a double-walled structure of the pressure vessel. In this instance, in order to produce a good heat insulating effect, the space in the double-walled structure must be evacuated to a vacuum higher than 0.01 Torr which is not easy to attain within the framwork of industry. By way of example, a Kiney single-stage hydraulic rotary vacuum pump which is largely used by industry has an evacuating capacity which tends to be lowered with an increase in the degree of vacuum. Specifically, when and after the degree of vacuum has attained about 0.05 Torr, the evacuating speed falls abruptly. Therefore, a relatively long time is required to create a vacuum of 0.01 Torr, posing a problem in terms of productivity.